1. Field of the Invention
The present invention relates to a load monitoring method and a load monitoring apparatus for monitoring a load imposed on a screw shaft of a kneading apparatus such as an extruder or a kneader.
2. Description of the Related Art
A kneading apparatus such as a kneader or an extruder includes a barrel having a long hollow portion in its axial direction, a screw shaft for kneading a material to be kneaded which is fed into the barrel, and a drive unit for rotating the screw shaft. The screw shaft has kneading blades for kneading the material to be kneaded.
In kneading, if the material to be kneaded is not in a thoroughly melted state or is difficult to be kneaded, a load larger than a designed strength may be imposed on the screw shaft. In conventional kneading apparatus, a load monitoring apparatus is used lest a load larger than a designed strength should be imposed on the screw shaft.
For example, in Japanese Patent Laid-Open Publication No. 2000-225641 there is disclosed an extruder having a load monitoring apparatus. In the load monitoring apparatus of the extruder disclosed in the patent literature, the number of revolutions of a drive unit and that of a screw shaft are detected by a revolution sensor attached to the drive unit and a revolution sensor attached to the screw shaft, respectively, and the difference between both revolutions is grasped as a load, and the load is monitored so as not to exceed a mechanical strength designed for the screw shaft. This conventional extruder is provided with a screw shaft protecting mechanism. According to this screw shaft protecting mechanism, when a load larger than the designed strength is imposed on the screw shaft, the rotational speed of the screw shaft is decreased or the amount of a to-be-kneaded material fed is decreased, thereby protecting the screw shaft.
In the extruder of the Japanese patent laid-open publication 2000-225641, however, no consideration is given to the case where a repetitive load of a degree not exceeding the designed mechanical strength is imposed on the screw shaft and causes fatigue fracture of the shaft. Therefore, even if operation is continued in such a manner that the load does not exceed the designed mechanical strength of the screw shaft, there is a fear that the screw shaft may be fractured by fatigue fracture.
On the other hand, in the extruder of the Japanese patent laid-open publication 2000-225641, the screw shaft can be protected by controlling the load to a value smaller than the designed mechanical strength of the screw shaft. However, if an attempt is made to set a load at a very small load, it is required to decrease the rotational speed of the screw shaft or decrease the amount of the to-be-kneaded material fed, thus resulting in lowering of the working efficiency or increase of the controlling frequency for the rotational speed or the amount of the material to be fed and causing a great variation in the amount of the material fed. Thus, there is a fear of the productivity being deteriorated markedly.